This invention relates in general to sewing machines and in particular to a new and useful thread cutting mechanism.
A thread cutting device is disclosed in German utility model 79 12 758. In this prior art device, a spring acts on the piston rod of the air cylinder by which the first step of motion of the thread catcher is supported and the opposite second step of motion is braked. The spring helps to overcome starting resistances while executing the first step of motion which may occur especially after a longer standstill. During the second step of motion, the spring reduces the speed of the thread catcher insofar as even thin threads can be pulled out without a risk of tearing and moved to the counter-knife, and satisfactory cutting is effected even under adverse conditions. The thread catcher is further equipped with a second air cylinder whose piston rod, with the thread catcher standing still, engages an opening of a member which is connected to the piston rod of the first air cylinder, thereby preventing the spring from pulling the thread catcher into the path of motion of the needle in instances of an air supply failure.
However, a satisfactory operation of the thread catcher requires additional, considerably expensive equipment, namely, aside from the spring, an adjusting device for presetting the spring tension, and the second air cylinder. Even more expensive is the mounting, since the initial tension of the spring must be adjusted as a function of the respective spring rate, and the position of the second air cylinder must exactly correspond to the mounted position of the first air cylinder, to enable it to perform its retaining function.